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Biological Changes, Over-fishing & Climate Change

The boarfish, Capros, is a deep water species from the continental shelf. It is an extremely rare occurrence on the Yorkshire coast.

The Yorkshire coast, fronting the rich waters of the central North Sea and the vast central bank of the Dogger, is many ways unique in the hydrological influences it receives, since it is both at the tail end of currents derived from the Gulf Stream via the North Atlantic Drift while at the same time also receives influence derived from the Channel influx. It thus receives planktonic and other organisms from warmer more exotic waters, together with rare fishes from the open Atlantic, with crops of species records not seen elsewhere because of this phenomenon. At the same time it receives southern species from the southern North Sea and via the Channel. A number of northern animals disappear at their southern most limit along the Yorkshire coast. The waters of the western central North Sea are therefore a most suitable place to try and quantify any effects of environmental change with respect to “global warming” or NAO induced effects. In the event of any creeping environmental changes taking place in British waters, its singular properties make it more likely that effects will be very noticeable here, and it would also therefore be of no surprise if great changes to established historic fisheries were also to take effect. Indeed the fisheries of the Yorkshire coast have altered radically since 1990 alongside noticeable faunistic changes, a few of which have been noticed by fishermen.

The difficulty is in identifying and quantifying what is an effect of over-fishing, and which if any, those of environmental parameters. However, it is beyond doubt that there have indeed been huge biological changes in this very unique and distinct sea area. Although not its original purpose, the survey had, from its initiation, the potential of revealing shifts in species frequency provided the daily recording was intense and constant enough and the co-operation of local fishermen could be kept going on a long-term basis.

After such a long period of recording the species frequenting this area, immense changes have been recorded among a number of invertebrate and fish taxa, and three in particular may be cited here, including the Red Mullet and the Velvet Swimming crab.There is one particular fish, however, known in Britain as the Blue Mouth Red-fish, and elsewhere in the world as the Rosebelly Sculpin, whose distribution has so radically changed above all others, that it has become a most important North Sea indicator, excluding the debatable cod, to demonstrate that there has indeed been a major climate-induced shift in the ecology of this marine species that hitherto fore had a clearly defined ecological niche, and from which juveniles have been disturbed, transported and dispersed with subsequently amazing results.

Distributional Change in the Blue Mouth Red-fish Helicolenus dactylopterus dactylopterus.
The fact that the biology and distribution of this fish have changed so dramatically and spectacularly must have implications in any view of what may or may not be happening to other marine species and community systems and the transport mechanism of larval and ‘o’ group fishes into and down the North Sea. The captures by surprised Galway fishermen and anglers put the species on the front page of Fishing News (May 28 2001), but that small news item belied the terrific change that had taken place with regard to the ecology of this fish during the previous decade, and which event suggests that the rise or fall in levels of other species may equally have been effected to some degree by this same mechanism.

Prior to 1986 there was no suggestion of any particular trend or long-term event taking place on the Yorkshire Coast, other than the deteriorating status of the cod stocks from the early 60s boom, and relevant warnings about this to the industry from the scientific community, except for a small but definite shift in the status of the Velvet Swimming Crab, which by 1990 had become a huge and exploitable population. However at the end of the 80s and early 90s an unprecedented crop of mostly extremely rare fishes associated with oceanic conditions were caught in the Yorkshire area.. These included specimens of the barrel-fish, Hyperoglyphe and the Cornish blackfish, Schedophilus, species that are particularly rare at any point in the British Isles, and remarkable records of the Boar fish, Capros, and the deep-water smooth-head, Alepocephalus; all fishes associated with deep-water or the continental slope, and transported in from the Atlantic.

At the same time, small juvenile specimens of the Blue Mouth, Helicolenus, began to appear over the North Sea grounds, but these were to prove to be no transitory invaders, they were to become permanent, reproductive residents to some of these newly colonised Yorkshire localities.

Marbled in red, pink and white, adults of this striking and sedentary fish attain great age and a length of more than 32 cm in some British localities. Until 1990 this fish remained associated with its typical textbook habitat, the continental slope of the western seaboard in depths over 200 metres, thence across the northern North sea to the deep water about Norway. The only adult breeding population south of this, at least known to this author, being of a colony of large fine fish confined to the deeper-water trench of the Moray Firth, just a few miles off Fraserborough. Its relatively deep water habitat also meant it was largely unknown to many British fishermen.

South of this, the species was entirely absent, and historically there are only a few odd records of its occurrence, including one fish that astounded the knowledgeable and pioneering fisheries biologist, E.W.L Holt, when it was caught in 5 fathoms near the Humber estuary during his period of work at the Grimsby docks. It was therefore not a species that was expected to be seen in the present survey, but when the juveniles, at obviously the minimum size the commercial trawls would retain, suddenly began to be taken in local catches they immediately became the subject of particular study, with fishermen at the port being encouraged to return every example they came across.

Succeeding years saw them being taken throughout the area and southwards even into the Wash. For a brief period the species was scattered and widespread, but with time it retracted to particularly colonise the deeper grounds including those off the Tees, the holes and pits about the Dogger, and off the Humber. It has even been taken in the shallow waters of the Bloden ground with its very different current system principally originating from the Channel. There is also some indication that at some pits near the Dogger, it has displaced to some extent the orange coloured Norway Haddock, Sebastes viviparous, with which some fishermen may confuse it, and which were formerly well known residents in those areas.

From the initial catches of small fishes in the early 1990’s, the insurgents have grown steadily year on year, the largest fishes at present always being male. Hundreds of specimens have now been returned over the years for biological sampling and age determination in a study that is ongoing. A number of live specimens were taken to Sea-Life Centres without any success in maintaining them, while three fish were retained alive by this author for more than two years, when they proved to be the most inactive of fish, perching frog-like, on the “ fingers” of the pectoral fins, often holding position without moving for days or weeks on end, and preferring to take prey only when it was practically on their snout.

The maturing ovaries of Helicolenus are gelatinous in appearance. After mating, sperm is stored in the ovaries and fertilisation occurs internally later in the cycle

The close size-composition of the catches, and the progressive increments in size in each succeeding year, demonstrates that the fish currently living in this area principally originated from the massive influx of tiny fish of two or three year-classes in the early 90s. Whatever physical abnormality of their environment caused such a massive displacement of the juveniles of this species, the ensuing conditions also ensured their survival, since this fish has now been resident in a huge and shallow sea area with which, most importantly, it had no previous biological association or success, and virtually no historical record.

Its new found success is further demonstrated by the fact that, as individuals quickly grew to maturity, spawning is believed to have first occurred in 1996, but most definitely in 1997. For the last few years, therefore, pelagic eggs and larval stages have entered the plankton in a sea area where they too had never been represented before.

It may be pointed out that the eggs of this species, in contrast to those of most other British fish, were unknown to science as little as thirty years ago, and indeed its strange reproductive biology has only been clarified in the last few years, despite commercial activity on this species in other parts of the world.

In the late 90’s, small three and four year old fish have begun to be caught again, for the first time since the original insurgents appeared, among the now older and mature fish. The sudden precipitation of juveniles into the North Sea from their deep-water niche of the Continental slope nearly two decades ago, is in some way associated with the NAO, and it is suggested that the previous appearance of these fish in these waters almost exactly one hundred years before, was due to the same mechanism. On that occasion, however, the effect must have been on a very much smaller scale and with nothing like the same number of individuals being transported into the central North Sea grounds; there was no subsequent success of colonisation and reproduction, the phenomenon witnessed during the 1990’s.

It remains to be seen whether the species will retain its hold on these new habitats in the future, and whether it can , in these localities, attain the great age and size of those living deeper down on the continental slope, but the continued presence and ageing of the original insurgents, and the appearance of new generations perhaps originating on their new North Sea spawning grounds, suggests that it will.

Changes in the Status of the Red Mullet, Mullus surmeletus

A landing of red mullet at Scarborough. During the early years of the survey this was a scarce North sea species.

Essentially a fish of southern waters, the Channel, and previously, in limited numbers in the eastern Southern North Sea, this fish formerly occurred in very small numbers elsewhere throughout the North Sea and Scottish waters, being generally regarded as rare in these areas. In the past, incidence for Yorkshire waters has always been low, in some years occurrences not even reaching double figures, although tiny juvenile fish, some 7/10 cm TL, were discovered during the survey, from infrequent findings in cod stomachs, to be frequenting the coastline, and were not being sampled by the trawl. This species too has shown a marked increase through the 1990’s with a brief yearly fishery for it developing early in the decade on a centre of increasing aggregation discovered at that time. Despite the targeting of this brief shoaling area in summer by numerous boats of various N.E.ports, together with French boats, the species continues to be an increasingly successful North Sea species. Fishing activity on this area was heavy in 2001 and included two Whitby boats landing some 1750 kilos of red mullet one morning in late June

During the last two decades, records of its occurrence on the Yorkshire coast have increased steadily to the point where it is now frequently the case that as many red mullet appear on just one days market as were previously accumulated in 2, 3 or more years of records in the early days of the survey.

Changes in the Status of the Velvet Swimming Crab

A common species occurring just offshore. It has a gelatinous appearance and a loose skin, and has a sucker disc on the belly between the fins, by which means it can fasten itself to the sea bed.

Although known from most areas around the British Isles, the population of this species on the Yorkshire coast could previously have been described as scarce or uncommon; in fact during a particular museum project in 1967 to obtain living specimens, it proved impossible to secure specimens, either from commercial crab-potting boats, most of whose crews had never seen a specimen, or from the shore; similarly, it was at that time also unknown to the local angling fraternity who regularly work the shore for soft-crab bait.

Since the early 1980’s there has been a complete change in the status of this crab on this coastline, where for a period it continued to proliferate year by year. Such a population explosion in a crustacean has not been experienced here by any other species at any previous time. For a while this crab began to displace the previously abundant green shore crab throughout the area, and it has become the dominant species on the Yorkshire shore, while offshore it is taken down to 40 metres.

So abundant has it become that occasionally some areas were avoided by inshore trawlers due to the immense quantities capable of being taken and the damage to be incurred on the nets. Although a goodly local fishery now exists for a species once so uncommon, the difficulties associated with handling and keeping have deferred anything other than comparatively minor landings at Scarborough.

A large John Dory of 1.8 kg from the central North sea. Fish of this size are rare in this sea area, most being juveniles of one or two years of age.

Whether the displacement of species other than the shore crab is occurring, for example on other species of swimming crab which are among the principle prey items of young cod, has not so far been noticed. Given the unprecedented and continuing explosion of this species to the present level, its position vis a vis the important local lobster stocks is of interest since a potential may exist for adverse competition with, or even predation of, lobster juveniles and their recruitment to the fishery. No such conflict has been observed however, and lobster stocks appear to be stronger than they have ever previously been.

Many other species changes have been noted including the welcome return of increasing numbers of halibut, Hippoglossus, and infrequent captures of the black-bellied angler, Lophiusbudegassa, another new species to the Yorkshire fauna, not seen in the central North Sea prior to the 1990s. The greater frequency of certain squids, including the “soft” squid Todaropsis eblanae from the north and Loligo vulgaris from the southern North Sea and Channel, both useful indicators of various water masses originating from different entries of this enclosed sea area, has also been notable. Other species, including the Bass, Dicentrarchus labrax, has also greatly increased in this sea area, to the point of now being capable of supporting a valuable winter fishery. Numbers of Twaite shad, Alosa fallax, have also increased, possibly associated with climate change and a greater northward, winter-time, migration, although another southern species, the Dory, Zeus faber, appears to have shown no marked change in frequency during the entire length of the survey. Some species, however, have become extinct in the area, and the populations of others greatly depressed, from their mid 1960’s status.